Strain-specific BDNF expression of rat primary astrocytes

Comprehensive characterization of the neurogenic and neuroprotective action of a novel TrkB agonist using mouse and human stem cell models of Alzheimer's disease

BACKGROUND

Neural stem cell (NSC) proliferation and differentiation in the mammalian brain decreases to minimal levels postnatally. Nevertheless, neurogenic niches persist in the adult cortex and hippocampus in rodents, primates and humans, with adult NSC differentiation sharing key regulatory mechanisms with development. Adult neurogenesis impairments have been linked to Alzheimer's disease (AD) pathology. Addressing these impairments by using neurotrophic factors is a promising new avenue for therapeutic intervention based on neurogenesis. However, this possibility has been hindered by technical difficulties of using in-vivo models to conduct screens, including working with scarce NSCs in the adult brain and differences between human and mouse models or ethical limitations.

METHODS

Here, we use a combination of mouse and human stem cell models for comprehensive in-vitro characterization of a novel neurogenic compound, focusing on the brain-derived neurotrophic factor (BDNF) pathway. The ability of ENT-A011, a steroidal dehydroepiandrosterone derivative, to activate the tyrosine receptor kinase B (TrkB) receptor was tested through western blotting in NIH-3T3 cells and its neurogenic and neuroprotective action were assessed through proliferation, cell death and Amyloid-β (Aβ) toxicity assays in mouse primary adult hippocampal NSCs, mouse embryonic cortical NSCs and neural progenitor cells (NPCs) differentiated from three human induced pluripotent stem cell lines from healthy and AD donors. RNA-seq profiling was used to assess if the compound acts through the same gene network as BDNF in human NPCs.

RESULTS

ENT-A011 was able to increase proliferation of mouse primary adult hippocampal NSCs and embryonic cortical NSCs, in the absence of EGF/FGF, while reducing Aβ-induced cell death, acting selectively through TrkB activation. The compound was able to increase astrocytic gene markers involved in NSC maintenance, protect hippocampal neurons from Αβ toxicity and prevent synapse loss after Aβ treatment. ENT-A011 successfully induces proliferation and prevents cell death after Aβ toxicity in human NPCs, acting through a core gene network shared with BDNF as shown through RNA-seq.

CONCLUSIONS

Our work characterizes a novel BDNF mimetic with preferable pharmacological properties and neurogenic and neuroprotective actions in Alzheimer's disease via stem cell-based screening, demonstrating the promise of stem cell systems for short-listing competitive candidates for further testing.

Microglia and BDNF at the crossroads of stressor related disorders: Towards a unique trophic phenotype

Stressors ranging from psychogenic/social to neurogenic/injury to systemic/microbial can impact microglial inflammatory processes, but less is known regarding their effects on trophic properties of microglia. Recent studies do suggest that microglia can modulate neuronal plasticity, possibly through brain derived neurotrophic factor (BDNF). This is particularly important given the link between BDNF and neuropsychiatric and neurodegenerative pathology. We posit that certain activated states of microglia play a role in maintaining the delicate balance of BDNF release onto neuronal synapses. This focused review will address how different "activators" influence the expression and release of microglial BDNF and address the question of tropomyosin receptor kinase B (TrkB) expression on microglia. We will then assess sex-based differences in microglial function and BDNF expression, and how microglia are involved in the stress response and related disorders such as depression. Drawing on research from a variety of other disorders, we will highlight challenges and opportunities for modulators that can shift microglia to a "trophic" phenotype with a view to potential therapeutics relevant for stressor-related disorders.

Dual mechanisms for the regulation of brain-derived neurotrophic factor by valproic acid in neural progenitor cells

Autism spectrum disorders (ASDs) are neurodevelopmental disorders that share behavioral features, the results of numerous studies have suggested that the underlying causes of ASDs are multifactorial. Behavioral and/or neurobiological analyses of ASDs have been performed extensively using a valid model of prenatal exposure to valproic acid (VPA). Abnormal synapse formation resulting from altered neurite outgrowth in neural progenitor cells (NPCs) during embryonic brain development has been observed in both the VPA model and ASD subjects. Although several mechanisms have been suggested, the actual mechanism underlying enhanced neurite outgrowth remains unclear. In this study, we found that VPA enhanced the expression of brain-derived neurotrophic factor (BDNF), particularly mature BDNF (mBDNF), through dual mechanisms. VPA increased the mRNA and protein expression of BDNF by suppressing the nuclear expression of methyl-CpG-binding protein 2 (MeCP2), which is a transcriptional repressor of BDNF. In addition, VPA promoted the expression and activity of the tissue plasminogen activator (tPA), which induces BDNF maturation through proteolytic cleavage. Trichostatin A and sodium butyrate also enhanced tPA activity, but tPA activity was not induced by valpromide, which is a VPA analog that does not induce histone acetylation, indicating that histone acetylation activity was required for tPA regulation. VPA-mediated regulation of BDNF, MeCP2, and tPA was not observed in astrocytes or neurons. Therefore, these results suggested that VPA-induced mBDNF upregulation was associated with the dysregulation of MeCP2 and tPA in developing cortical NPCs.

Caregiver maltreatment causes altered neuronal DNA methylation in female rodents

Negative experiences with a caregiver during infancy can result in long-term changes in brain function and behavior, but underlying mechanisms are not well understood. It is our central hypothesis that brain and behavior changes are conferred by early childhood adversity through epigenetic changes involving DNA methylation. Using a rodent model of early-life caregiver maltreatment (involving exposure to an adverse caregiving environment for postnatal days 1-7), we have previously demonstrated abnormal methylation of DNA associated with the brain-derived neurotrophic factor (Bdnf) gene in the medial prefrontal cortex (mPFC) of adult rats. The aim of the current study was to characterize Bdnf DNA methylation in specific cell populations within the mPFC. In the prefrontal cortex, there is approximately twice as many neurons as glia, and studies have recently shown differential and distinctive DNA methylation patterns in neurons versus nonneurons. Here, we extracted nuclei from the mPFC of adult animals that had experienced maltreatment and used fluorescence-activated cell sorting to isolate cell types before performing bisulfite sequencing to estimate methylation of cytosine-guanine sites. Our data indicate that early-life stress induced methylation of DNA associated with Bdnf IV in a cell-type and sex-specific manner. Specifically, females that experienced early-life maltreatment exhibited greater neuronal cytosine-guanine methylation compared to controls, while no changes were detected in Bdnf methylation in males regardless of cell type. These changes localize the specificity of our previous findings to mPFC neurons and highlight the capacity of maltreatment to cause methylation changes that are likely to have functional consequences for neuronal function.

Inhibition of glial proliferation, promotion of axonal growth and myelin production by synthetic glycolipid: A new approach for spinal cord injury treatment

PURPOSE

After spinal cord injury (SCI) a glial scar is generated in the area affected that forms a barrier for axon growth and myelination, preventing functional recovery. Recently, we have described a synthetic glycolipid (IG20) that inhibited proliferation of human glioma cells. We show now that IG20 inhibited the proliferation of astrocytes and microglial cells, the principal cellular components of the glial scar, and promoting axonal outgrowth and myelin production in vitro.

METHODS

Glial cells were inhibited with IG20 (IC50≈10 μM) and studied by RT-PCR, Western Blotting, immunoprecipitation and fluorescence microscopy. Axonal outgrowth in dorsal root ganglia (DRG) and myelin production by oligodendrocytes were analyzed by immunocytochemistry. Adult rats were assayed in spinal cord contusion model and the recovery of treated animals (n = 6) and controls (n = 6) was followed.

RESULTS

The IG20 was localized in the cytosol of glial cells, forming a complex with RhoGDIα, a regulator of RhoGTPases. Treatment of astroglial cultures with IG20 increase the expression of BDNF receptor genes (TrkBT1, TrkB Full). IG20 reduced the astroglial marker GFAP, while increasing production of myelin basic protein in oligodendrocytes and promoted axonal outgrowth from DRG neurons. Local injection of IG20, near a spinal cord contusion, promoted the recovery of lesioned animals analyzed by BBB test (P <  0.05).

CONCLUSIONS

We propose that inhibition of astrocytes and microglia by IG20 could be diminished the glial scar formation, inducing the re-growth and myelination of axons, these elements constitute a new approach for SCI therapy.

Resveratrol induces the expression of interleukin-10 and brain-derived neurotrophic factor in BV2 microglia under hypoxia

Microglia are the resident macrophages of the central nervous system (CNS) and play an important role in neuronal recovery by scavenging damaged neurons. However, overactivation of microglia leads to neuronal death that is associated with CNS disorders. Therefore, regulation of microglial activation has been suggested to be an important target for treatment of CNS diseases. In the present study, we investigated the beneficial effect of resveratrol, a natural phenol with antioxidant effects, in the microglial cell line, BV2, in a model of hypoxia injury. Resveratrol suppressed the mRNA expression of the pro-inflammatory molecule, tumor necrosis factor-α, and promoted the mRNA expression of the anti-inflammatory molecule, interleukin-10, in BV2 microglia under hypoxic conditions. In addition, resveratrol inhibited the activation of the transcription factor, nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB), which is upstream in the control of inflammatory reactions in hypoxia-injured BV2 microglia. Moreover, resveratrol promoted the expression of brain-derived neurotrophic factor (BDNF) in BV2 microglia under hypoxic stress. Overall, resveratrol may promote the beneficial function of microglia in ischemic brain injury.

Individual differences in novelty seeking predict subsequent vulnerability to social defeat through a differential epigenetic regulation of brain-derived neurotrophic factor expression

Some personality traits, including novelty seeking, are good predictors of vulnerability to stress-related mood disorders in both humans and rodents. While high-novelty-seeking rats [high responders (HRs)] are vulnerable to the induction of depressive-like symptoms by social defeat stress, low-novelty-seeking rats [low responders (LRs)] are not. Here, we show that such individual differences are critically regulated by hippocampal BDNF. While LR animals exhibited an increase in BDNF levels following social defeat, HR individuals did not. This difference in hippocampal BDNF expression promoted the vulnerability of HR and the resilience of LR rats. Indeed, preventing activation of BDNF signaling by infusing the BDNF scavenger TrkB-Fc into the dentate gyrus of the hippocampus of LR rats led to social defeat-induced social avoidance, whereas its activation in HR rats by the TrkB agonist 7,8-dihydroxyflavone promoted social approach. Along with the changes in BDNF expression following defeat, we report in LR animals a downregulation of the inactive BDNF receptor TrkB.T1, associated with an activation of CREB through Akt-mediated signaling, but not MSK1-mediated signaling. In HR animals, none of these molecules were affected by social defeat. Importantly, the BDNF upregulation involved an epigenetically controlled transcription of bdnf exon VI, associated with a coherent regulation of relevant epigenetic factors. Altogether, our data support the importance of hippocampal BDNF regulation in response to stressful events. Moreover, we identify a specific and adaptive regulation of bdnf exon VI in the hippocampus as a critical regulator of stress resilience, and strengthen the importance of epigenetic factors in mediating stress-induced adaptive and maladaptive responses in different individuals.

Rosmarinic acid ameliorates depressive-like behaviors in a rat model of CUS and Up-regulates BDNF levels in the hippocampus and hippocampal-derived astrocytes

Rosmarinic acid (RA), a primary constituent of a Chinese herbal medicine, has been shown to have some therapeutic effects in an animal model of depression, but its underlying mechanisms are poorly understood. Sprague-Dawley rats were exposed to chronic unpredictable stress (CUS) for 21 days, and received RA for 14 days from the last week of CUS, then the behavioral changes, hippocampal pERK1/2 and BDNF levels were observed. Rats were further treated with U0126 (an ERK1/2 phosphorylation inhibitor) 30 min before RA treatment to assess the effects of RA and ERK1/2 signaling in depressive-like behavior and hippocampal BDNF levels. In addition, brains of newly born Sprague-Dawley rats were used to harvest and expand hippocampal astrocytes. Cells were exposed to different concentrations of RA (sham, 1, 5, 10, 20, and 40 μg/mL) or U0126 (2 μM as a final concentration) + RA (sham, 1, 5, 10, 20, and 40 μg/mL) for 48 h, and the pERK1/2 and BDNF levels were assessed by western and ELISA assays. RA administration (10 mg/kg daily) reversed depressive-like behaviors in rats exposed to a chronic unpredictable stress paradigm and restored pERK1/2 protein expression and hippocampal brain-derived neurotrophic factor (BDNF). Moreover, in vitro experiments revealed that 20 μg/mL RA increased pERK1/2 and BDNF levels in cultured astrocytes. Interestingly, the effects of RA were inhibited by U0126. RA might be a useful treatment for depression and the changes in ERK1/2 signaling and BDNF levels may play a critical role in the pharmacological action of RA.

Brain-derived neurotrophic factor interacts with astrocytes and neurons to control respiration

Respiratory rhythm is generated and modulated in the brainstem. Neuronal involvement in respiratory control and rhythmogenesis is now clearly established. However, glial cells have also been shown to modulate the activity of brainstem respiratory groups. Although the potential involvement of other glial cell type(s) cannot be excluded, astrocytes are clearly involved in this modulation. In parallel, brain-derived neurotrophic factor (BDNF) also modulates respiratory rhythm. The currently available data on the respective roles of astrocytes and BDNF in respiratory control and rhythmogenesis lead us to hypothesize that there is BDNF-mediated control of the communication between neurons and astrocytes in the maintenance of a proper neuronal network capable of generating a stable respiratory rhythm. According to this hypothesis, progression of Rett syndrome, an autism spectrum disease with disordered breathing, can be stabilized in mouse models by re-expressing the normal gene pattern in astrocytes or microglia, as well as by stimulating the BDNF signaling pathway. These results illustrate how the signaling mechanisms by which glia exerts its effects in brainstem respiratory groups is of great interest for pathologies associated with neurological respiratory disorders.

Resveratrol Produces Neurotrophic Effects on Cultured Dopaminergic Neurons through Prompting Astroglial BDNF and GDNF Release

Increasing evidence indicated astroglia-derived neurotrophic factors generation might hold a promising therapy for Parkinson's disease (PD). Resveratrol, naturally present in red wine and grapes with potential benefit for health, is well known to possess a number of pharmacological activities. Besides the antineuroinflammatory properties, we hypothesized the neuroprotective potency of resveratrol is partially due to its additional neurotrophic effects. Here, primary rat midbrain neuron-glia cultures were applied to investigate the neurotrophic effects mediated by resveratrol on dopamine (DA) neurons and further explore the role of neurotrophic factors in its actions. Results showed resveratrol produced neurotrophic effects on cultured DA neurons. Additionally, astroglia-derived neurotrophic factors release was responsible for resveratrol-mediated neurotrophic properties as evidenced by the following observations: (1) resveratrol failed to exert neurotrophic effects on DA neurons in the cultures without astroglia; (2) the astroglia-conditioned medium prepared from astroglia-enriched cultures treated with resveratrol produced neurotrophic effects in neuron-enriched cultures; (3) resveratrol increased neurotrophic factors release in the concentration- and time-dependent manners; (4) resveratrol-mediated neurotrophic effects were suppressed by blocking the action of the neurotrophic factors. Together, resveratrol could produce neurotrophic effects on DA neurons through prompting neurotrophic factors release, and these effects might open new alternative avenues for neurotrophic factor-based therapy targeting PD.

Imipramine induces brain-derived neurotrophic factor mRNA expression in cultured astrocytes

Depression is one of the most prevalent and livelihood-threatening forms of mental illnesses and the neural circuitry underlying depression remains incompletely understood. Recent studies suggest that the neuronal plasticity involved with brain-derived neurotrophic factor (BDNF) plays an important role in the recovery from depression. Some antidepressants are reported to induce BDNF expression in vivo; however, the mechanisms have been considered solely in neurons and not fully elucidated. In the present study, we evaluated the effects of imipramine, a classic tricyclic antidepressant drug, on BDNF expression in cultured rat brain astrocytes. Imipramine dose-dependently increased BDNF mRNA expression in astrocytes. The imipramine-induced BDNF increase was suppressed with inhibitors for protein kinase A (PKA) or MEK/ERK. Moreover, imipramine exposure activated transcription factor cAMP response element binding protein (CREB) in a dose-dependent manner. These results suggested that imipramine induced BDNF expression through CREB activation via PKA and/or ERK pathways. Imipramine treatment in depression might exert antidepressant action through BDNF production from astrocytes, and glial BDNF expression might be a target of developing novel antidepressants.

Resveratrol promotes neurotrophic factor release from astroglia

Neurotrophic factors such as glial cell line-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) are considered to contribute to the development, maintenance and survival of neurons, glia and oligodendrocytes. Astroglia are a major source of various neurotrophic factors. Thus, enhancement of astroglia-mediated neurotrophic factor release might hold promising potential for neurological diseases. Resveratrol, a natural non-flavonoid polyphenol found in grapes and red wine, has been recognized to be beneficial for health. Here, rat primary astroglia-enriched cultures were used to investigate the effects of resveratrol-mediated neurotrophic factor release and the related mechanisms. The cultures were treated with 25–100 μmmol/L resveratrol for 12–48 h. Results showed resveratrol increased BDNF and GDNF production in the culture medium. In addition, the production of BDNF in the supernatant of cultures was increased five-fold over control cultures 24 h after resveratrol treatment and then remained high 36 h later. Meanwhile, the production of GDNF was initially increased by up to four-fold 24 h after resveratrol treatment and continued to increase to six-fold at 36 h and remained at a high level till 48 h. Western blot analysis of BDNF and GDNF protein in astroglia at different time points after resveratrol treatment indicated similar increases. Furthermore, resveratrol significantly induced the phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2) and cAMP responsive element-binding protein (CREB) in astroglia. Overall, resveratrol is effective in promoting astroglia-derived neurotrophic factor release, and this effect is mediated, at least in part, by the activation of ERK1/2 and CREB.